I got it working with help. I asked in binhex's vpn app thread. He said to try adding a bunch of other ports to the VPN_INPUT_PORTS and it MIGHT work
VPN_INPUT_PORTS:
-p 1900:1900/udp \
-p 3005:3005 \
-p 5353:5353/udp \
-p 8324:8324 \
-p 32410:32410/udp \
-p 32412:32412/udp \
-p 32413:32413/udp \
-p 32414:32414/udp \
-p 32469:32469
I think I just entered them as 1900,3005,5353, etc. I also added port forwarding for 32400 in my router.
The iptable command line you entered has to be done from the plex container in docker. Click on your plex container in docker and select console, then add it. I was adding it to the unraid terminal, but didnt work until I tried it in the plex docker console. I connect to the vpn the same way you do, with the -net=container method.
Hopefully this helps.